What is web-performance?

A guide for making web pages load and respond faster by improving images, fonts, code bundles, and rendering. It uses Core Web Vitals: measurements of loading speed, interaction response, and layout movement.

Use it to optimize images, fonts, JavaScript bundles, network loading, and rendering. It also supports performance checks during frontend design audits.

It helps find and fix the causes of a slow-feeling interface and performance regressions. The measure-first approach shows which changes improve real user experience.

web-performance — Skill for Claude Code by sukirman1901

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods

npx agentmods add skills/sukirman1901/frontend-expert/web-performance

Any agent

npx skills add sukirman1901/frontend-expert --skill web-performance

Clone the repo

git clone --depth 1 https://github.com/sukirman1901/frontend-expert

Made for: Claude Code.

Or install frontend-expert, the plugin that ships this one along with the rest of its 25 skills, 5 commands, 3 agents, 2 hooks.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/skills/sukirman1901/frontend-expert/web-performance.svg)](https://agentmods.dev/skills/sukirman1901/frontend-expert/web-performance)

Your own site

<a href="https://agentmods.dev/skills/sukirman1901/frontend-expert/web-performance"><img src="https://agentmods.dev/badge/skills/sukirman1901/frontend-expert/web-performance.svg" alt="Measured on agentmods" height="20"></a>

Per session 74 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 1,639 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. Scan, not verified.

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5.1	$0.00074	$0.01639
Opus 5	$0.00037	$0.00820
Sonnet 5	$0.00015	$0.00328
Haiku 4.5	$0.00007	$0.00164

Measured 6d ago against content hash 8a83e7d9185a, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-06, from the pricing page.

Security

Grade A, and why

web-performance scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 6d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

skills/web-performance/SKILL.md · 198 lines

How it starts

The opening of the file, as written. The whole thing — 198 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Web Performance

Overview

Measure-first performance for user-facing UI. Hit Core Web Vitals targets; prefer compositor-friendly work and sensible loading.

When to Use

Performance regressions or budgets
Deep /design audits
Image/font/bundle optimization work
User reports "slow", "lemot", "loading lama"

Targets

Metric	Target	Measures
LCP	≤ 2.5s	Largest visible content element render time
INP	≤ 200ms	Interaction to visual response time
CLS	≤ 0.1	Unexpected layout shift score

Decision trees

Image format decision

Is it a photo or complex image?
  YES → Is AVIF supported by your target browsers?
         YES → AVIF (smallest, best quality)
         NO  → WebP (wide support, good compression)
  NO  → Is it an icon or simple graphic?
         YES → SVG (scalable, tiny) or Reicon
         NO  → Is transparency needed?
               YES → WebP or PNG (WebP preferred)
               NO  → WebP or JPEG (WebP preferred)

Font loading strategy

Is the font critical for LCP text?
  YES → Preload + font-display: swap
        <link rel="preload" as="font" href="/font.woff2" type="font/woff2" crossorigin>
  NO  → Is it a secondary/accent font?
        YES → font-display: swap (load async)
        NO  → Is it used only on specific pages?
              YES → Lazy load via JS / @font-face on that page only
              NO  → font-display: optional (may skip if slow)

LCP resource priority

What is the LCP element?
  IMAGE → fetchpriority="high" + preload + NO lazy-load + width/height set
  TEXT  → Preload the font + inline critical CSS for that heading
  VIDEO → Preload poster image + fetchpriority="high" on poster
  
Is there a render-blocking script above LCP?
  YES → Move to bottom / add defer / async
  NO  → Good

Workflow

1. Loading

Preconnect critical origins
Preload LCP resources (<link rel="preload">)
fetchpriority="high" on LCP image — never lazy-load it
Self-host fonts with font-display: swap and woff2 format
Modern image formats (WebP/AVIF) with <picture> fallback
Code-split routes and heavy features (lazy() + Suspense)

Read the full file on GitHub · 198 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

6d ago First seen · 198 lines · 74 tokens per session scan A 8a83e7d9185a

Subscribe to this mod's changes

web-performance is a skill published in the GitHub repository sukirman1901/frontend-expert (10 stars, last pushed 3d ago), licensed MIT. It adds 74 tokens to every session and 1,639 once invoked, about $0.0004 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.

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